The present invention relates to signal processing and more particularly to a use of all-pass filtering to correct the phase of speakers in a speaker system to improve performance in a cross-over region.
Modern sound systems have become increasingly capable and sophisticated. Such systems may be utilized for listening to music or integrated into a home theater system. One important aspect of any sound system is the speaker suite used to convert electrical signals to sound waves. An example of a modern speaker suite is a multi-channel 5.1 channel speaker system comprising six separate speakers (or electroacoustic transducers) namely: a center speaker, front left speaker, front right speaker, rear left speaker, rear right speaker, and a subwoofer speaker. The center, front left, front right, rear left, and rear right speakers (commonly referred to as satellite speakers) of such systems generally provide moderate to high frequency sound waves, and the subwoofer provides low frequency sound waves. The allocation of frequency bands to speakers for sound wave reproduction requires that the electrical signal provided to each speaker be filtered to match the desired sound wave frequency range for each speaker. Because different speakers, rooms, and listener positions may influence how each speaker is heard, accurate sound reproduction may require to adjusting or tuning the filtering for each listening environment.
Cross-over filters (also called bass-management filters) are commonly used to allocate the frequency bands in speaker systems. Because each speaker is designed (or dedicated) for optimal performance over a limited range of frequencies, the cross-over filters are frequency domain splitters for filtering the signal delivered to each speaker.
Common shortcomings of known cross-over filters include an inability to achieve a net or recombined amplitude response, when measured by a microphone in a reverberant room, which is sufficiently flat or constant around the cross-over region to provide accurate sound reproduction. For example, a listener may receive sound waves from multiple speakers such as a subwoofer and satellite speakers, which are at non-coincident positions. If these sound waves are substantially out of phase (viz., substantially incoherent), the waves may to some extent cancel each other, resulting in a spectral notch in the net frequency response of the audio system. Alternatively, the complex addition of these sound waves may create large variations in the magnitude response in the net or combined subwoofer and satellite response. Additionally, bass-management filters for each speaker, which are typically nonlinear phase Infinite Impulse Response (IIR) filters (for example, Butterworth design), may further introduce complex interactions during the additive process.
Room equalization has traditionally been approached as a classical inverse filter problem for compensating the magnitude responses, or for performing filtering in the time domain to obtain a desired convolution between a Room Transfer Function (RTF) and the equalization filter. Specifically, for each of the equalization filters, it is desired that the convolution of the equalization filter with the RTF, measured between a speaker and a given listener position, results in a desired target equalization curve. From an objective perspective, the target equalization curve is represented in the time domain by the Kronecker delta function. However, from a psychoacoustical perspective, subjectively preferred target curves may be designed based on the dimensions of the room and the direct to reverberant energy in the measured room response. For example, the THX® speaker system based X-curve is used as a target curve and movie theaters.
Although equalization may work well in simulations or highly controlled experimental conditions, when the complexities of real-world listening environments are factored in, the problem becomes significantly more difficult. This is particularly true for small rooms in which standing waves at low frequencies may cause significant variations in the frequency response at a listening position. Furthermore, since room responses may vary dramatically with listener position, room equalization must be performed, in a multiple listener environment (for example, home theater, the movie theater, automobile, etc.), with measurements obtained at multiple listening positions. Known equalization filter designs, for multiple listener equalization, have been proposed which minimizes the variations in the RTF at multiple positions. However, including an equalization filter for each channel for a single listener or multiple listeners, will not alleviate the issue of complex interaction between the phase of the non-coincident speakers, around the cross-over region, especially if these filters introduce additional frequency dependent delay.